This invention relates to a novel process for the removal of permanganate reducing compounds and alkyl iodides formed by the carbonylation of methanol in the presence of a Group VIII metal carbonylation catalyst. More specifically, this invention relates to a novel process for reducing and/or removing precursors of permanganate reducing compounds and alkyl iodides from intermediate streams during the formation of acetic acid by said carbonylation processes.
Among currently employed processes for synthesizing acetic acid one of the most useful commercially is the catalyzed carbonylation of methanol with carbon monoxide as taught in U.S. Pat. No. 3,769,329 issued to Paulik et al on Oct. 30, 1973. The carbonylation catalyst comprises rhodium, either dissolved or otherwise dispersed in a liquid reaction medium or else supported on an inert solid, along with a halogen containing catalyst promoter as exemplified by methyl iodide. The rhodium can be introduced into the reaction system in any of many forms, and it is not relevant, if indeed it is possible, to identify the exact nature of the rhodium moiety within the active catalyst complex. Likewise, the nature of the halide promoter is not critical. The patentees disclose a very large number of suitable promoters, most of which are organic iodides. Most typically and usefully, the reaction is conducted with the catalyst being dissolved in a liquid reaction medium through which carbon monoxide gas is continuously bubbled.
An improvement in the prior art process for the carbonylation of an alcohol to produce the carboxylic acid having one carbon atom more than the alcohol in the presence of a rhodium catalyst is disclosed in commonly assigned U.S. Pat. Nos. 5,001,259, issued Mar. 19, 1991; 5,026,908, issued Jun. 25, 1991 and 5,144,068, issued Sep. 1, 1992 and European patent 161,874 B2, published Jul. 1, 1992. As disclosed therein acetic acid is produced from methanol in a reaction medium comprising methyl acetate, methyl halide, especially methyl iodide, and rhodium present in a catalytically effective concentration. The invention therein resides primarily in the discovery that catalyst stability and the productivity of the carbonylation reactor can be maintained at surprisingly high levels, even at very low water concentrations, i.e. 4 weight (wt) % or less, in the reaction medium (despite the general industrial practice of maintaining approximately 14 wt % or 15 wt % water) by maintaining in the reaction medium, along with a catalytically effective amount of rhodium, at least a finite concentration of water, methyl acetate and methyl iodide, a specified concentration of iodide ions over and above the iodide content which is present as methyl iodide or other organic iodide. The iodide ion is present as a simple salt, with lithium iodide being preferred. The patents teach that the concentration of methyl acetate and iodide salts are significant parameters in affecting the rate of carbonylation of methanol to produce acetic acid especially at low reactor water concentrations. By using relatively high concentrations of the methyl acetate and iodide salt, one obtains a surprising degree of catalyst stability and reactor productivity even when the liquid reaction medium contains water in concentrations as low as about 0.1 wt %, so low that it can broadly be defined simply as xe2x80x9ca finite concentrationxe2x80x9d of water. Furthermore, the reaction medium employed improves the stability of the rhodium catalyst, i.e. resistance to catalyst precipitation, especially during the product recovery steps of the process wherein distillation for the purpose of recovering the acetic acid product tends to remove from the catalyst the carbon monoxide which in the environment maintained in the reaction vessel, is a ligand with stabilizing effect on the rhodium. U.S. Pat. Nos. 5,001,259, 5,026,908 and 5,144,068 are herein incorporated by reference.
It has been found that a low water carbonylation process for the production of acetic acid reduces such by-products as carbon dioxide, hydrogen, and propionic acid. However, the amount of other impurities, present generally in trace amounts, is also increased, and the quality of acetic acid sometimes suffers when attempts are made to increase the production rate by improving catalysts, or modifying reaction conditions.
These trace impurities affect quality of acetic acid, especially when they are recirculated through the reaction process. Among the impurities, which decrease the permanganate time of the acetic acid, are carbonyl compounds, and unsaturated carbonyl compounds. As used herein, the phrase xe2x80x9ccarbonylxe2x80x9d is intended to mean compounds which contain aldehyde or ketone functional groups which compounds may or may not possess unsaturation. See Catalysis of Organic Reaction, 75, 369-380 (1998), for further discussion on impurities in a carbonylation reaction system.
The present invention is directed to reduction and/or removal of permanganate reducing compounds (PRC""s) such as acetaldehyde, acetone, methyl ethyl ketone, butyraldehyde, crotonaldehyde, 2-ethyl crotonaldehyde, and 2-ethyl butyraldehyde and the like, and the aldol condensation products thereof. It also leads to reduction of propionic acid. Reduction of other impurities include alkyl iodides such as ethyl iodide, propyl iodide, butyl iodide, pentyl iodide, hexyl iodide, and the like.
It is desirable to remove alkyl iodides from the reaction product since traces of these impurities (in the acetic acid product) tend to poison the catalyst used in the production of vinyl acetate, the product most commonly produced from acetic acid. The present invention is thus also directed to removal of alkyl iodides, in particular C2-12 alkyl iodide compounds. The carbonyl impurities may further react with iodide catalyst promoters to form multi-carbon alkyl iodides, e.g., ethyl iodide, butyl iodide, hexyl iodide and the like. Since many impurities originate with acetaldehyde, it is therefore a primary objective to remove or reduce the acetaldehyde and alkyl iodide content in the reaction system.
Conventional techniques to remove impurities include treatment of acetic acid with oxidizers, ozone, water, methanol, activated-carbon, amines, and the like, which treatment may or may not be combined with distillation of the acetic acid. The most typical purification treatment involves a series of distillations of the final product. It is known to remove carbonyl impurities from organic streams by treating the organic streams with an amine compound such as hydroxylamine which reacts with the carbonyl compounds to form oximes followed by distillation to separate the purified organic product from the oxime reaction products. However, the additional treatment of the final product adds cost to the process and it has been found that distillation of the treated acetic acid product can result in additional impurities being formed.
While it is possible to obtain acetic acid of relatively high purity, the acetic acid product formed by the above described low water carbonylation process and purification treatment, frequently remains deficient with respect to the permanganate time. This is due to the presence therein of small proportions of residual impurities. Since a sufficient permanganate time is an important commercial test, which the acid product must meet for many uses, the presence therein of such impurities that decrease permanganate time is objectionable. The removal of minute quantities of these impurities from the acetic acid by conventional treatment and distillation techniques is not economically or commercially feasible by distillation since the impurities have boiling points close to that of the acetic acid product.
It is important to determine where in the carbonylation process impurities can be removed. It is also important to determine by what economically viable process impurities can be removed without risk of further contamination to the final product or unnecessary added costs. JP patent application 5-169205 discloses a method for manufacture of high purity acetic acid by adjusting the acetaldehyde concentration of the reaction solution below 1500 ppm. By maintaining the acetaldehyde concentration in the reaction solution below 1500 ppm, it is stated that it is possible to suppress the formation of impurities and manufacture high purity acetic acid by performing only basic distillation operations during purification of the crude acetic acid formed.
EP 487,284, B1, published Apr. 12, 1995, states that carbonyl impurities present in the acetic acid product generally concentrate in the overhead from the light ends column. Accordingly, the light ends column overhead is treated with an amine compound i.e., hydroxylamine which reacts with the carbonyl compounds to allow such carbonyls to be separated from the remaining overhead by distillation, resulting in an acetic acid product which has improved permanganate time.
EP 0 687 662 A2 describes a process for producing high purity acetic acid whereby an acetaldehyde concentration of 400 ppm or less is maintained in the reactor by removal thereof using a single or multi-stage distillation process. Streams suggested for processing to remove acetaldehyde include a light phase comprising primarily water, acetic acid and methyl acetate; a heavy phase comprising primarily methyl iodide, methyl acetate and acetic acid; an overhead stream comprising primarily methyl iodide and methyl acetate; or a recirculating stream comprising the light and heavy phase combined. Although four streams are suggested for processing, the reference teaches and exemplifies use of the heavy phase. No teaching or suggestion is given regarding which stream(s) possesses the greatest concentration of acetaldehyde.
Also disclosed in EP""662 is management of reaction conditions to control the formation of acetaldehyde in the reactor. By controlling the formation of acetaldehyde, it is stated that reduction of by-products such as crotonaldehyde, 2-ethylcrotonaldehyde, and alkyl iodides are reduced. However, it is pointed out that management of reaction conditions xe2x80x9chave a defect to increase a by-production speed of propionic acid.xe2x80x9d indicating that propionic acid is a problem with the disclosed process of ""662.
Hence, EP""662 describes optimization of reaction conditions to avoid formation of acetaldehyde as well as removal of any acetaldehyde beyond a level of 400 ppm formed in the reactor.
While the above-described processes have been successful in removing carbonyl impurities from the carbonylation system and for the most part controlling acetaldehyde levels and permanganate time problems in the final acetic acid product, further improvements can still be made. There remains a need to determine where in the carbonylation process the permanganate reducing compounds, and in particular, acetaldehyde and alkyl iodides are most concentrated and therefore can be reduced or removed so as to insure consistent purity of product. At the same time, there remains a need to provide a process for reduction/removal of such carbonyl materials and iodide compounds without sacrificing the productivity of the carbonylation process or without incurring substantial additional operating costs.
It has now been discovered that a light ends phase from the light ends distillation column contains carbonyl-containing permanganate reducing compounds, and in particular acetaldehyde which may be further concentrated and removed from the process. In one aspect of this invention, the light ends phase is distilled twice, once through a distillation column which serves to separate the acetaldehyde, methyl iodide, and methyl acetate from acetic acid and water. The second distillation column serves to separate acetaldehyde from methyl iodide and methyl acetate and essentially serves to concentrate and purge the acetaldehyde from the process. Optionally, in another aspect of the invention, the resulting distillate from the second distillation is directed to an extractor to separate out concentrated acetaldehyde and return a residual saturated organic iodide solution to the carbonylation reaction system.
In another aspect of the invention, alkyl iodide compounds, in particular C2-12, may be removed or significantly reduced employing the described dual distillation process.
It has been found that when shutting down the carbonylation system, in particular the distillation columns employed in the present process, polymers of acetaldehyde, in particular higher molecular weight polymers tend to form and build up in the base of the second column. Another aspect of the present invention describes a method to deal with this problem. It has been found that a constant flow of solvent to maintain contact between the stream within the second distillation column and a solvent from an internal stream (such as one that contains a large percentage of acetic acid or methyl acetate) results in a polymer-free column base upon shut down of the unit. By having the base devoid of polymer build up, one may shut down and subsequently start up the column in a relatively trouble free, efficient, and cost-effective manner.
The present invention utilizes a light phase, which is an internal, intermediate stream in the process, instead of a heavy phase (as suggested in EP""662), for reduction, or removal of PRC""s, their precursors and alkyl iodide compounds. The art traditionally employs a heavy phase for treatment or removal of carbonyl impurities and in particular, removal of acetaldehyde. To date, the art was not aware-that light phase was the better option compared to the heavy phase to reduce PRC""s and alkyl iodides. Generally, the art employs an extractor before the second distillation; it has been found that the use of an extractor after the second distillation (or a post-extractor process) results in greater removal of acetaldehyde. It has also been found that due to the dual distillation process, coupled with the post extractor, a very concentrated acetaldehyde stream with essentially no methyl iodide is purged from the process. It has been found that the formation of meta- and paraldehyde in the second column, as well as higher molecular weight polymers thereof, can be inhibited or suppressed by the use of an internal stream comprising approximately 70 wt % water and 30 wt % acetic acid. Because the stream is internal, to the process, it does not place an added water load to the process. It has further been found that the recycle of the first column""s residue to the light ends column decanter can be used to extract more PRC""s from the heavy phase into the light phase and thus improve acetic acid product quality overall.
A preferred embodiment of the present invention is directed towards a process for reduction and/or removal of permanganate reducing compounds, their precursors, and C2-12 alkyl iodide compounds formed in the carbonylation of methanol to a product of acetic acid, wherein said methanol is carbonylated in a suitable liquid phase reaction medium comprising a Group VIII metal catalyst, an organic iodide and iodide salt catalyst promoter; the products of said carbonylation are separated into a volatile phase comprising product, and a less volatile phase comprising Group VIII metal catalyst, acetic acid, and iodide catalyst promoter; said product phase distilled in a distillation tower to yield a purified product and an overhead comprising organic iodide, methyl acetate, water, acetic acid, and unreacted methanol, directing at least a portion of the overhead to an overhead receiver decanter which separates the overhead into a light phase, comprising acetic acid and water, and a heavy phase comprising methyl acetate and organic iodide; and recycling the heavy phase to the carbonylation reactor, the improvement which comprises
(a) directing the light phase comprising acetic acid and water to a distiller which separates the mixture into two streams: residue stream (1) comprising primarily water and acetic acid, and overhead stream 2) comprising methyl iodide, methyl acetate, methanol, C 2-12 alkyl iodides, and permanganate reducing compounds(PRC""s);
(b) circulating stream (1) of step (a) to further processing and ultimately back to the reaction system, and stream (2) of step (a) to a second distiller which serves to strip the PRC""s and alkyl iodides from the mixture;
(c) optionally, forwarding the over head stream containing PRC""s or precursors thereof of step (b) to an extractor and,
(d) separating out concentrated PRC""s and alkyl iodides for disposal and returning the organic iodide phase of (b) or (c) as a stream containing a lower percentage of PRC""s, precursors, and/or C2-12 alkyl iodides to the carbonylation reaction system.
The bulk of the overhead from the light phase is recycled to the reactor. Thus, in accordance with the present invention, the inventory of PRC""s including acetaldehyde, and alkyl iodides is greatly reduced by this multiple distillation plus optional post extraction process and, at the same time, accomplishing such product quality without substantially increasing the cost of production.
It has been found that PRC""s, in particular acetaldehyde, crotonaldehyde, and 2-ethyl crotonaldehyde, and alkyl iodides, in particular hexyl iodide, are reduced by at least 50%, usually greater than that, employing the inventive process. Additionally, propionic acid has been reduced, usually greater than 20%, most often greater than 30% and 40%, and total iodides have been reduced by a percentage reduction of about 50%, most often greater than 60%. The permanganate time has been observed to increase by a percentage of about 50%, usually greater than 70% with the inventive process.
Once the inventive process was operational and shut down of the system was on going, it was discovered that polymers of acetaldehyde, in particular, polymers having a molecular weight greater than about 1000, tended to build up in the second column and plug the column. The polymers were found to be viscous, and thixotropic and tended to adhere to the walls of the column. Upon heating, these polymers tended to crystallize and harden along the walls of the column, making them very difficult to remove. It was found that this problem could be avoided by contacting the stream flowing through the second distillation column with solvent stream flow in an amount sufficient and at a flow rate sufficient to avoid aldol condensation polymer formation or to avoid formation of polymers of acetaldehyde. The solvent may be selected from acetic acid, methyl acetate, methanol, water, methyl iodide and the like or combinations thereof with acetic acid being preferred in view of the abundance of an internal stream to utilize. Generally, amounts sufficient to avoid aldol condensation reactions from occurring are rates of about 0.25-5 gallon per minute (gpm), preferably about 0.5-2 gpm with most preferable rate being about 1 gpm. It is undesirable to use an excess of solvent since this places a greater load on the system to reprocess the excess solvent. Although various positions of ingress of the solvent are acceptable, it is preferred that the solvent be contacted with the stream in the second distillation column at the base of the column.